1. Field of the Invention
The present invention relates to a diamond film synthesizing apparatus and a method thereof using a direct current glow discharge plasma enhanced chemical vapor deposition, and in particular to an improved diamond film synthesizing apparatus and a method thereof using a direct current glow discharge plasma enhanced chemical vapor deposition advantageously providing a plurality of spaced-apart cathodes for forming a relatively large and strong plasma section, so that a high deposition rate can be obtained in large area, wherein each of the cathodes is connected to an improved cathode holders via a cathode connecting rod capable of preventing a formation of a solid carbon on the cathode, which disadvantageously affects stable production of a plasma by maintaining the temperature of the cathode at over 2100.degree. C.
2. Description of the Conventional Art
Conventionally, a direct current glow discharge plasma enhanced chemical vapor (CVD) deposition is well known as one of typical diamond synthesizing technologies among conventional diamond synthesizing apparatuses and methods. Plasma CVD is generally classified into two methods: a low temperature plasma enhanced method and a high temperature plasma enhanced method. The low temperature plasma enhanced method comprises a hot filament CVD method and a microwave plasma enhanced CVD method. The high temperature plasma enhanced CVD method includes an arc jet plasma CVD method.
In the low plasma enhanced method, the desired deposition rate can be obtained; meanwhile, the deposition rate slows down to 1 .mu.m/h, so it is difficult to increase the deposition rate due to the limited deposition rate. In the high temperature plasma enhanced CVD method, the deposition rate is relatively fast up to a 10-100 .mu.m/h; meanwhile, the deposition rate is limited within a diameter of 2-3 cm. In addition, there is a big problem in obtaining a uniform diamond film.
Meanwhile, in case of the direct current glow discharge plasma enhanced CVD method, since the characteristic of a plasma is similar to the higher plasma enhanced CVD method, a high deposition rate of 70 .mu.m/h can be obtained, and the construction thereof is simple, and the cost of electric power is low since it uses a direct current electric power source which is much cheaper than a microwave or a high frequency current power source.
Referring to FIG. 1, there is shown a conventional direct current glow discharge plasma enhanced CVD apparatus. The construction of the conventional direct current glow discharge plasma enhanced CVD apparatus will now be explained.
To begin with, there is provided a sealed cylindrical reactor 1 having an upper wall 1a and a lower wall 1b. A cathode 2 is half inserted into the upper wall 1a of the reactor 1 in a form that one end of which is extended into the reactor 1 and the other end of which is externally connected to a power supplier 3, one end of which being connected to the mound. An anode 4 is also half inserted into the lower wall 1b of the reactor 1, the upper portion of which is extended into the reactor 1 and the bottom portion of which is externally connected to the ground. Here, a substrate 5 is attached on the top of the anode 4.
Meanwhile, a pressure gauge 6 is disposed on a predetermined portion of the reactor 1. A gas supplier 7 is disposed on a predetermined portion of the reactor 1 for supplying gas thereinto. A vacuum pump 8 is disposed on a predetermined portion of the reactor 1 for creating a vacuum inside of the reactor 1.
Referring to FIG. 2A, there is shown a conventional cathode. The upper wall of a cathode 2 is cooled by water 10 flowing from an inlet port 9a to an outlet port 9b. Referring to FIG. 2B, there is shown a graph showing a temperature variation along a cathode. As shown therein, the heat generated at the bottom of the cathode 2 is transferred the top thereof. That is, a temperature of 2100.degree. C. at the bottom thereof gradually tapers down to room temperature level as the heat is transferred toward the top thereof.
The operation of the conventional direct current glow discharge plasma enhanced CVD apparatus will now be explained with reference to FIG. 1 and FIGS. 2A and 2B.
To begin with, the inside of the reactor 1 is evacuated by the vacuum pump 8; thereafter and the inside of the reactor 1 is charged with a gas source supplied by the gas supplier 7. In the above state, a direct current voltage is supplied to both the cathode 2 and the anode 4; thereafter, a plasma stream is generated between the cathode 2 and the anode 3, whereby a diamond synthesizing operation is performed on the surface of the substrate 6.
Generally, in the diamond synthesizing operation using a direct current glow discharge plasma enhanced CVD method, a cathode plays a most important key role for generating a stable plasma stream in a reactor. In addition, a cathode is commonly made of a metal, having a relatively high melting temperature, such as Mo, W, Ta, which are generally manufactured in a shape of a rod or filament. When a cathode made of such a metal is used, the temperature at the end portion of the cathode reaches up to about 2100.degree. C. At this time, a solid carbon, which is generally formed at a temperature of 1900.degree. C., is formed at the intermediate portion of the cathode because the temperature of 2100.degree. C. generated at the end portion of the cathode becomes a range of 1900.degree. C.-2100.degree. C. thereat. As the synthesizing operation is continued, the solid carbon gradually builds up, so that in the long run the formation of the solid carbon reaches to the end portion of the cathode, thereby covering the end portion of the cathode and thus the desired plasma can not be obtained. In addition, since the formation of the solid carbon builds up unevenly on the circumferential surface of the cathode and the end portion thereof, the current flow is biased, so that the cathode is partially heated and, thus, an arc is generated between the cathode and the substrate, discontinuing deposition of diamond.
Meanwhile, in a conventional current glow discharge plasma enhanced CVD method for synthesizing diamond, one cathode 2 is used as shown in FIG. 1. In this case, to increase the deposition area for generating the desired amount of plasma, the size of a cathode should be increased; however, as the cathode area increases, it is difficult to maintain the distance between the cathode and the substrate constant at every point. This makes the plasma shift to a position where the distance between the cathode and the substrate is minimum. Such a plasma shift makes a thickness of a diamond film uneven. Moreover, the deposition rate of diamond film is about 5 .mu.m/h, relatively low.
In addition, current density, a key element in synthesizing speed and film quality, is limited to a predetermined level, therefore, the larger the deposition area, the lower the deposition rate and the quality.